The present invention relates to a light emitting element drive device. Specifically, the light emitting element drive device can protect a circuit from an abnormal state by detecting an overcurrent while driving a light-emitting diode (LED) as a light emitting element.
A light emitting element, such as an LED, has been used as a lighting system and as a backlight for a display device, such as a liquid crystal panel. A protection circuit is provided at a circuit for driving the light emitting element in consideration of a malfunction of the light emitting element. For example, Japanese Patent Publication No. JP H09-331017 discloses a protection circuit that controls an electric current flowing through a light emitting element based on a voltage drop in an overcurrent detection resistor connected between a power source and the light emitting element.
Japanese Patent Publication No. JP 2006-222377 discloses a protection circuit that restrains an overcurrent, which flows when a short circuit occurs in an LED, by an electric current setting element. In this protection circuit, when the short circuit occurs in the LED, the electric current setting element that is connected in series to the LED restrains current increment so that power loss is reduced. In JP 2006-222377, a transistor is used as the electric current setting element. A collector of the transistor is connected to the LED. An emitter of the transistor is connected to a grounded part of a power source through a resistor. A base of the transistor is connected to an output of an operational amplifier. A connection point between the emitter of the transistor and the resistor is connected to an input of the operational amplifier. A reference voltage is connected to another input of the operational amplifier. Further, when a detection unit detects the short circuit of the LED, the reference voltage applied to the operational amplifier is lowered. As a result, since an output voltage of the operational amplifier is lowered when the short circuit of the LED occurs, an increase in electric current that flows in the transistor is restrained. Therefore, the power loss of the transistor that functions as the electric current setting element can be reduced.
However, because the protection circuit of JP H09-331017 has a complicated circuit for controlling the electric current that flows in the light emitting element, the number of parts increases.
The operational amplifier and a reference signal that controls an electric current flowing in the circuit are required for the protection circuit of JP 2006-222377. Therefore, there are the following problems. Increasing the number of parts complicates the circuit configuration. In addition, a pulse width modulation (PWM) dimming means is not disclosed in JP 2006-222377. When the transistor that is connected in series to the LED is turned ON/OFF to perform the PWM dimming by applying an external signal, the detection unit that detects the current increment caused by the short circuit of the LED is required to have excellent responsiveness. When the detection unit including the operational amplifier has poor responsiveness, there is a possibility that a malfunction occurs in the protection circuit. Thus, because the performance characteristic of the protection circuit decreases, there is a concern that the protection circuit cannot deliver superior protection performance.